1. Field of the Invention
The present invention relates to a technology for emitting an X-ray while changing the helical pitch so as to obtain transmission data for a subject to be examined, reconstructing an image of the subject to be examined from the obtained transmission data.
2. Description of the Related Art
The X-ray CT apparatus emits X-rays, detects X-rays transmitted through the subject to be examined, and reconstructs the image of the inside of the subject to be examined from projection data indicating the intensity of detected X-rays. It plays an important role in medical practices such as diagnosing diseases, treatment, planning surgery, and so forth. This X-ray CT apparatus has a variety of imaging methods, depending on the type of diagnostic imaging, including the method of administering a contract agent to the subject to be examined and then obtaining the image and the method of reconstructing the image in synchronization with the biological signal, for example. In any case, it is essential to obtain an image that is accurate enough for diagnostic imaging and to reconstruct the same.
In a method of administering a contract agent to the subject to be examined before obtaining the image, a pre-scan is performed, and then a scan is initiated after detecting that the contract agent has flowed into the area of which the image is to be reconstructed by this pre-scan. For contract-agent imaging of X-ray CT apparatus, it is essential to perform a scan while the contract agent administered to the subject to be examined has flowed into the reconstruction area of the image. Therefore, a pre-scan is repeated before the scan. This pre-scan scans part of the reconstruction area of an image with a lower radiation dose than the actual scan and determines concentration of contract agent flowing into the reconstruction area of an image depending upon the CT value of the obtained image. Once concentration of the contract agent reaches a certain point, the scan is initiated. The method of automatically determining the concentration of the contract agent by way of a predetermined threshold value and then automatically initiating the scan is referred to a “real prep-scan” (e.g., cf. Japanese Patent laid-open No. 2003-245275).
This real-prep scanning by a conventional X-ray CT apparatus is shown in FIG. 1. As shown in FIG. 1, first, an X-ray tube is positioned for a pre-scan at a point within the reconstruction area of the image, and then X-rays is emitted at the pre-scan position Z0 to obtain a pre-scan image. This pre-scan image is a tomographic image. Once the pre-scan image is obtained, the CT value of this image is calculated, and if it is at the threshold value or greater, the scan can be initiated.
For the scan, first, the bed is moved so that X-ray tube is positioned outside the reconstruction area of the image, and then a margin is set outside the starting edge of the reconstruction area of the image. After the margin is set, the scan is initiated from this margin area toward the ending edge of the reconstruction area of the image by the helical scan. The reason to set the margin is that X-rays for the number of views (BPview) required for reconstruction of the image is also emitted at the starting edge position of the image reconstruction by a helical scan. This is because, when the scan is initiated from the pre-scan position Z0 directly to the ending edge of the reconstruction area of the image, it is sometimes impossible to ensure the emission of X-rays for BPview at the starting edge position of the image reconstruction, which is often outside the pre-scan position Z0.
For the real prep-scan, it is essential to scan at the time when the contract agent administered to the subject to be examined is flowing into the reconstruction area of the image. Therefore, it is important to perform the scan immediately after the pre-scan detects that the contract agent has flowed into the area. This is because, if the contract agent flows out of the reconstruction area of the image or is lost and the necessary concentration of the contract agent thus cannot be obtained, an image with superior quality cannot be reconstructed. However, as mentioned above, for a conventional real prep-scan, when the pre-scan detects the flow of the contract agent, X-ray tube must be placed outside the reconstruction area of the image, the margin must be formed, and the scan must then be initiated from this margin. In short, the initiation of the scan for the reconstruction area of the image must be suspended from the time of detection of flowing of the contract agent due to this margin setting and the scan of the margin. Accordingly, with a conventional real prep-scan, an image with sufficient accuracy is not necessarily reconstructed. Furthermore, X-rays is emitted at the area that corresponds to the margin as well, so the subject to be examined will be affected by exposure even when he is outside the reconstruction area of the image, which is also not desirable for the subject to be examined.
Moreover, diagnostic imaging includes an approach to diagnose only the region of interest and an approach to diagnose a larger area e.g. searching the affected area from the entire body. For such an approach to diagnose the larger area, the reconstruction area of the image may include an area involving physical movement such as from the heart, lungs, and so forth, and an area without such physical movement. The physical movement corresponds to the movement of activities of the body system. Between the area involving physical movement and areas without physical movement, the method of obtaining data required to reconstruct the image for that area is different.
For areas involving physical movement of the heart, lungs, and so forth, a biological signal synchronization reconstruction method that is characteristic of the method of obtaining data required for reconstruction is used (e.g., cf. Japanese Patent laid-open No. 2005-66042).
According to the biological signal synchronization reconstruction method, biological signal data showing changes in the physical movement is obtained in synchronization with the imaging of a subject to be examined, and then, partial data obtained while obtaining the biological signal data showing movement of a particular phase is extracted from projection data and gathered, and then the gathered data is used to reconstruct the image. The helical pitch is limited to the pitch that passes the area involving an area of physical movement when multiple cycles of a changing biological signal are repeated, in order to gather multiple sets of data showing the physical movement of the particular phase.
Accordingly, to image an area that includes an area involving movement and an area without movement, it has been necessary to perform a separate scan by segmenting the reconstruction area of the image into an area involving movement and an area without movement, because methods of obtaining projection data required for reconstructing the image are different.
FIG. 2 is a view showing a method of imaging a conventional area including an area involving physical movement and an area without physical movement. The reconstruction area SE, which involves physical movement and is reconstructed in synchronization with the biological signal, is included in the entire reconstruction area of the image and is situated between reconstruction areas NSE that do not involve physical movement and are reconstructed in asynchronization with the biological signal. In this case, the first scan images one of the reconstruction areas NSE that is reconstructed in asynchronization with the biological signal. Second, it images the reconstruction area SE that is reconstructed in synchronization with the biological signal by a second scan. Furthermore, it images another reconstruction area NSE that is reconstructed in asynchronization with the biological signal by a third scan.
Then, it reconstructs the projection data obtained by the first and third scans in asynchronization with physical movement into the volume image, and also reconstructs the projection data obtained by the second scan into the volume image by the biological signal synchronization reconstruction method.
In addition, there is sometimes a demand to display an image of areas that are obtained as a whole so as to observe the inside of the subject to be examined body overall. This is, for example, the case when contract agent is administered to the subject to be examined in order to observe the overall blood circulation.
However, when there is both a reconstruction area SE that involves physical movement and a reconstruction area NSE without physical movement in the area to be imaged, in order to image these areas separately, there is a time lag between the image of the reconstruction area SE involving physical movement and the image of the reconstruction area NSE without physical movement. As a result, the concentration of the contract agent appears uneven, depending on the area being observed. This makes it difficult to totally diagnose from the imaged areas, even if such areas are displayed as a whole. Furthermore, separate imaging of these areas means that the subject to be examined must be exposed to risk from radiation multiple times, which is not desirable. On the other hand, if the reconstruction area SE involving physical movement and the reconstruction area NSE without physical movement are imaged in a single simultaneous scan, and the resultant projection data is then not reconstructed by way of a biological signal synchronization reconstruction method, this leads to inaccurate data results around the area involving physical movement, thereby significantly decreasing the quality of the image.